The present invention relates to a spread spectrum communication system, and more particularly, to a spread spectrum communication system which does not need to comprise a spread code generation circuit at a receiving (demodulating) side for generating a despreading pseudonoise, but comprises an automatic gain control (AGC) means most suitable thereto.
According to the spread spectrum communication system, a carrier is primary modulated by an information signal, and the primary modulated carrier undergoes secondary modulation by a wide band noise-like spread code so as to spread it over a very wide bandwidth. Various spread spectrum communication systems are known such as a direct sequence (DS) system, frequency hopping (FH) system, hybrid system and the like, depending upon the secondary modulation scheme. The present invention beyonds to the DS system. A spread spectrum communication system has a number of advantages as in the following:
(1) High security and privacy protection; PA0 (2) High immunity to external interference and noise, and to intentional disturbance; PA0 (3) Compatibility with conventional communication systems; PA0 (4) No control station and control channel are required unlike a multichannel access (MCA) station; PA0 (5) Capability of identifying station by using address codes; PA0 (6) Seemingly no radio signal transmission in the spectrum due to low power distribution of the DS system (Capability of transmitting radio signal with thinly spread power); PA0 (7) Capability of increasing a number of stations in the given band with trade-off of a slight degradation of speech quality; and PA0 (8) Capability of multiplex operation in the same frequency band by changing pseudo noise codes.
Taking the above advantages into account, the spread communication systems are now widely applied not only to commercial communications but also to consumer use.
Referring to FIGS. 1 and 2, there will be described the fundamental principle of spread spectrum communication system having an AGC circuit at the receiving side. FIG. 1 is a block diagram showing the basic arrangement of a conventional spread spectrum communication system according to a DS system, and FIG. 2 shows spectrum waveforms at various circuit portions of the conventional system shown in FIG. 1.
As shown in FIG. 1, a station A as a transmission side comprises a primary modulator 1 which primarily modulates a carrier with an information signal and outputs a primary modulated signal F.sub.1 shown in FIG. 2(a), a spread code generator 2 for generating spread code signal F.sub.SS shown in FIG. 2(b) such as pseudo noise (PN) codes, a spread modulation circuit 3 which secondarily modulates the primary modulated signal F.sub.1 by the spread codes supplied from the generator 2, and an antenna A.sub.1 for output of a transmission signal F.sub.1S after amplification by secondary modulation of the circuit 3. The type of primary modulations is not particularly limited, but frequency modulation (FM), phase shift keying (PSK) and the like may be adopted (in this specification, the description is assumed to use PSK modulation). For the secondary modulation (spread modulation), PSK modulation is generally performed using pseudo noise (PN) code. It is required for the PN code to be like random noise as much as possible, and to have a predetermined code period for detecting the code at the receiving side.
Numeral 4 represents a general station transmitting a radio wave F.sub.n as an interference wave.
Next, a station B as the receiving side comprises an antenna A2 for receiving signals F.sub.1S to F.sub.3S and F.sub.n, an AGC circuit 5 for setting the signal F.sub.1S after filtering and amplification by a filter and high-frequency amplifier at the predetermined level, a despread (spread) code generator 6 for generating spread codes to be used for despreading the received signal, a despread (spread) circuit 7 for despreading the received signal from the AGC circuit 5 in accordance with the spread codes, a filter 8 for passing the frequency components of preferably a specific narrow band of an output from the despreading circuit 7, and a demodulator 9 for demodulating an output from the filter 8 and outputting an information signal.
The despread code generator 6 is in synchronization with the spread code generator 2 of the station A, and the same PN code (F.sub.SS) is used. Radio waves incoming to the antenna A.sub.2 include not only the radio wave F.sub.1S but also radio waves F.sub.2S, F.sub.3S, . . . , from other SS stations and the radio wave F.sub.n from the general station other than the other SS stations, as shown in FIG. 2(c). In order to explain briefly, the function will be described when the radio wave F.sub.n is simultaneously received with the radio wave F.sub.n from the general station 4.
The spectrum shown in FIG. 2(a) is restored from the desired radio wave F.sub.1S shown in FIG. 2(d) by means of despreading the wave F.sub.1S by the despread circuit 7, and at the filter (the narrow band pass filter is required) 8, components other than F.sub.1S (refer to FIG. 2(e)). Then, original information signal is demodulated at the demodulator 9 is removed. As seen from FIG. 2(e), the output signal from the filter 8 includes not only the waveform F.sub.1S but also a fraction of the interference radio wave F.sub.n. The ratio of the desired signal power to the residual power of the interference (less residual power is preferred) is called the DN ratio (a ratio of desired signal power to interference power). In order to obtain a larger DN ratio, the spread bandwidth should advantageously be as wide as possible. The spread bandwidth is generally set at several hundreds to several thousands of times as wide as the frequency bandwidth of the information signal to be transmitted.
The basic principle of the spread spectrum communication system has been described before. Next, the particular operation will be theoretically described for primary and secondary modulation and demodulation during the spread spectrum communication. A spread spectrum signal S(t) (F.sub.1S shown in FIG. 1) for spread spectrum communication is given by the following equation, taking the information data as "d(t)[+1 or -1]", the spread code F.sub.SS as "P(t)[+1 or -1]", and the carrier as "cos.omega..sub.c t": EQU S(t) =d(t).multidot.P(t).multidot.cos.omega..sub.c t . . . (1)
(where .omega..sub.c =2.pi.fc)
At the receiving side, the incoming spread spectrum signal S(t) is transformed into a two-phase PSK signal such as "d(t).multidot.cos.omega..sub.c t" by multiplying (or correlating, despreading) it by the spread code P(t). The spread code P(t) (actually P(t) with a slight delay) is obtained, the spread code P(t) is principly synchronized with the spread code associated with the spread spectrum signal at the time of transmission, by using a spread code clock derived from the incoming spread spectrum signal. The obtained two-phase PSK signal is multiplied by the reproduced carrier "cos.omega..sub.c t" (actually cos.omega.w.sub.c t with slight delay) for synchronous detection to obtain as follows: EQU d(t)(cos.omega..sub.c t).sup.2 =d(t) (1+cos2.omega..sub.c t)/2.
After removing the double carrier component "2.omega..sub.c t" by the filter, the information data d(t) are demodulated.
The process gain G.sub.P of the spread spectrum communication is given by: EQU G.sub.P =B.sub.P /B.sub.D . . . (2)
where B.sub.D denotes the bandwidth (main lobe of the spectrums) of the two-phase PSK signal "d(t)cos.omega..sub.c t", and B.sub.P denotes the bandwidth (main lobe of the spectrums) of the spread spectrum signal spread by the spread code P(t). The process gain G.sub.P is several hundreds to several thousands for the case of an ordinary design. Suppression of interference signals, noises and the like depends upon the process gain. Thus, the wider the frequency bandwidth of the spread spectrum signal is set to the information data d(t), the larger the improvement of the interference signals, noises and the like that can be obtained. Such improvement therefore depends substantially and exclusively upon the process gain G.sub.P.
In the spread spectrum communication system, "despreading" at the receiving side is the most important, but it is difficult to generate a reliable spread code required in performing such despreading. Despreading methods presently adopted in general include a despreading method using an AFC loop, delay locked loop and multiplier, and a despreading method using a synchronous loop of matched filter and multiplier.
However, the above-described despreading methods for the conventional spread spectrum communication system, pose the problem of having a complicated circuitry, and problems associated with circuit adjustment and cost effectiveness. These problems must be solved for allowing various applications to and developments in consumer devices and apparatus.
Furthermore, the conventional communication system has the configuration in which the AGC circuit 5 is arranged at the preceding stage of the despread circuit 7 in order to maintain stable operation when the system is especially adapted to the device and apparatus for field use. If there are other signals having a stronger field strength than the spread spectrum (SS) signals to be received, it is difficult to properly control the SS system for the signals because the AGC circuit 5 responses to these other signals.